FBG sensor interrogation method using semiconductor optical amplifier in ring cavity configuration

ABSTRACT

A sensor device that uses a number of bragg grating (FBG) sensors and novel interrogation system with a ring cavity configuration for simultaneous time-division-multiplexex (TDM) and wavelength-division-multiplexed (WDM) interrogation of FBG sensors. The ring cavity includes an amplifier, and output coupler and an optical circulator. The coupler is connected to a wavelength measuring system and the optical circulator is connected to the FBG sensors. The FBG sensors can be in a number of groups. TDM interrogation is applied to each group of FBG sensors while WDM interrogation is applied to each FBG sensors within each group.

FIELD OF THE INVENTION

The invention relates to time- and wavelength-division multiplexinginterrogation system of fiber Bragg grating sensors. More particularly,it relates to an optical interrogation system based on an opticalamplifier in ring cavity configuration.

BACKGROUND OF THE INVENTION

Fiber Bragg grating (FBG) has been accepted as an important sensortechnology because of its self-referencing capability, large-scalemultiplexing capability and immunity to electromagnetic interference.Wavelength-division multiplexing (WDM) technique [1] can be easilyemployed to multiplex and interrogate FBG sensor array, and thus it iscommonly used in FBG sensor applications. The number of FBG sensors thatcan be accommodated in a WDM-FBG sensor array is determined by theusable spectral bandwidth of the system and the wavelength-shift of eachFBG sensor. On the other hand, time-division-multiplexing (TDM)technique identifies the sensing signal by gating the pulses reflectedfrom FBGs, therefore, FBGs having identical resonant wavelengths can bedeployed along the same fiber. Hence, TDM-FBG technique relieves thespectral bandwidth issue and permits the interrogation of up to 100 FBGsalong a fiber. However, the reflectivity of the FBGs employed in TDMsensor systems are generally less than 5% and thus the reflected signalpower is fairly weak in comparison with WDM-FBG systems.

Various TDM systems have been reported during the last decade [2-7]. Themain challenge of a TDM system is to measure the sensing signalaccurately because of the weak signal power reflected from lowreflectivity sensors. Amplified spontaneous emission (ASE) generated byan erbium-doped fiber amplifier [3] and passively mode-locked fiberlaser operating in square-pulse regime [4] have been employed as sourcesto illuminate low reflectivity FBGs array with the objective to increasethe signal power reflected from the FBGs. Another approach utilizedactive mode-locking technique [5] to selectively address individual FBGin a two-FBG array that act as reflectors of a linear cavityerbium-doped fiber laser. This active laser approach produces intenseoutput power at the resonant wavelength of the selected FBG sensor.Recently, Lloyd et al. [6-7] reported a resonant TDM configuration thatuses a SOA, a broadband reflector and an array of 10 FBGs to construct alinear resonant cavity sensor system, high power and high extinctionratio output signal were demonstrated by properly gating the SOA. Whilethose prior practices are workable to various degrees, the need stillexists for a simpler FBG sensor system with better performance and lessconstruction expense.

REFERENCE

[1] A. D. Kersey, M. A. Davis, H. J. Patrick, M. LeBlanc, K. P. Koo, C.G. Askins, M. A. Putnam, and E. J. Friebele, “Fiber grating sensors,” J.Lightw. Tecnhol., vol. 15, pp. 1442-1463, August 1997.

[2] D. J. F. Cooper, T. Coroy, and P. W. E. Smith, “Time-divisionmultiplexing of large serial fiber-optic Brag grating sensor arrays,”Appl. Opt., vol. 40, pp. 2643-2654, 2001.

[3] T. A. Berkoff, M. A. Davis, D. G. Bellemore, A. D. Kersey, G. M.Williams, and M. A. Putnam, “Hybrid time and wavelength divisionmultiplexed fiber Bragg grating sensor array,” in Proc. SPIE SmartSensing, Processing and Instrumentation, vol. 2444, pp. 288-294, 1995.

[4] M. A. Putnam, M. L. Dennis, I. N. Duling III, C. G. Askins, and E.J. Friebele, “Broadband square-pulse operation of a passivelymode-locked fiber laser for fiber Bragg grating interrogation,” Opt.Lett., vol. 23, pp. 138-140, 1998.

[5] A. D. Kersey and W. W. Morey, “Multiplexed Bragg grating fibre-laserstrain-sensor system with mode-locked interrogation,” IEE Electron.Lett., vol. 29, pp. 112-114, January 1993.

[6] G. D. Lloyd, I. Bennion, L. A. Everall, and K. Sugden, “Novelresonant cavity TDM demodulation scheme for FBG sensing,” in Proc. CLEO'04, paper CWD4, May 2004.

[7] G. D. Lloyd, L. A. Everall, K. Sugden, and I. Bennion, “Resonantcavity time-division-multiplexed fiber Bragg grating sensorinterrogator,” IEEE Photon. Technol. Lett., vol. 16, pp. 2323-2325,October 2004.

SUMMARY OF THE INVENTION

One object of the present invention is therefore to provide a simplerFBG sensor interrogation system with high performance. To achieve theobject, an embodiment of the present invention utilizes a ring cavityconfiguration as exemplified in FIG. 1. The ring cavity configuration,compared with the liner cavity configuration, offers the advantages offewer component counts (lower cost), lower insertion loss (betterperformance), and simpler driving pattern (less complexity).

According to one aspect of the invention, there is provided a sensorsystem, comprising a plurality of FBG sensors; a ring cavity comprisinga gain modulated device having an input end and an output end and anoptical coupler; and a wavelength measuring system. The gain modulateddevice may be driven by a pulse generator.

According to another aspect of the invention, in a sensor system shownin FIG. 6, the above mentioned optical coupler has at least four opticalleads connected to the input end of the gain modulated device, theoutput end of the gain modulated device, the wavelength measuring systemand the FBG sensors, respectively. In this system, the optical coupleris used both for outputting signal to the sensors and the wavelengthmeasuring system and for feeding the reflected signal from the sensorsinto the ring cavity.

According to another aspect of the invention, a sensor system furthercomprises an optical circulator with at least three ports that connectto the optical coupler, the input end of the gain modulated device andthe FBG sensors, respectively. In this sensor system the optical coupleris not used for feeding the reflected signal from the sensors to thering cavity and thus may need only three optical leads, connected to theoutput end of the gain modulated device, the wavelength measuring systemand said FBG sensors, respectively.

According to another aspect of the invention, a sensor system furthercomprises an isolator between the optical coupler and the wavelengthmeasuring system and a second isolator between the optical circulator orcoupler (if there is no circulator) and the input end of the gainmodulated device (GMD). In the circulator-less configuration (see FIG.6), the coupler feeds the signal to the wavelength measuring system andto the sensor array, while the signal reflected from the sensors alsofeedback to the cavity (the output end of the GMD and input end ofisolator) through this coupler, the isolator ensures that light in thecavity propagates in clockwise direction (unidirectional) inside thering cavity.

According to another aspect of the invention, the gain modulated devicein a sensor system may be a semiconductor amplifier or linear opticalamplifier. The gain modulated device may have two coupled components: anErbium doped fiber amplifier and an electro-optical modulator.

According to another aspect of the invention, a sensor system may havetwo or more gain modulated devices to increase the usable wavelengthbandwidth.

According to another aspect of the invention, a sensor system maycontain a number of FBG sensors which are divided into a number ofunits. Sensors within a given unit may have different resonantwavelengths and are suitable for wavelength-division-multiplexinginterrogation. On the other hand, sensors from different units shouldhave substantially identical resonant wavelengths as their interrogationis of time-division-multiplexing.

According to another aspect of the invention, there is provided a methodfor interrogating a number of sensor units each containing one or moreFBG sensors. The method comprises:

-   (a) switching on and off a gain modulated device (GMD) at a    repetition rate predetermined for interrogating one of the sensor    units. The GMD has separate input and output ends for receiving    input and sending output;-   (b) splitting the output from the GMD into at least two portions;-   (c) sending one of the portions to a wavelength measuring system;-   (d) sending another portion to the sensor units each of which    generate a reflection;-   (e) feeding the reflection by the sensor units as input to the input    end of the GMD which is configured to be switched on when the    reflection from one sensor unit arrives while being switched off    when the reflection from any other sensors arrives;-   (f) repeating steps (b)-(e) for a number of times, which may be    performed not in the order from (a) to (e) and some of the steps may    be performed simultaneously, until the reflection from the    particular sensor is sufficiently amplified for measurement;-   (g) changing the repetition rate of step (a) to a predetermined rate    suitable for interrogating another sensor unit and repeating steps    (b)-(e); and-   (h) repeating step (g) a number of times so that all the sensor    units are interrogated.

The various features of novelty which characterize the invention arepointed out with particularity in the claims annexed to and forming apart of this disclosure. For a better understanding of the invention,its operating advantages, and specific objects attained by its use,reference should be made to the drawings and the following descriptionin which there are illustrated and described certain embodiments of theinvention.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a sensor system configuration according to an embodiment ofthe present invention.

FIG. 2 shows a sensor system of FIG. 1 where a semiconductor amplifieris used as GMD.

FIG. 3 shows a sensor system of FIG. 1 where a linear optical amplifieris used as GMD.

FIG. 4 shows a sensor system of FIG. 1 where an Erbium doped fiberamplifier coupled with an electro-optical modulator is used as GMD.

FIG. 5 shows a sensor system of FIG. 1 where two semiconductoramplifiers are used as GMD to widen the wavelength region of the system.

FIG. 6 shows another sensor system configuration according to anotherembodiment of the present invention.

FIG. 7 shows power and extinction ratio of the output signal as afunction of additional attention as measured for a sensor system of thepresent invention and wherein the inset shows the output spectra versusattenuation settings.

FIG. 8 represents output spectra of a number of FBG sensors measured atthe output of the ring cavity each at a different driving frequency ofthe pulse generator and the inset shows the wavelength change of theeighth FBG as a function of applied strain.

FIG. 9 shows the output spectra of four groups of three FBGs measured atthe output of the ring cavity.

FIG. 10 provides the equations for calculating the driving frequency(f_(i) and pulse-width (X) for using the sensor system of the presentinvention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

System Configuration and Operation Principles:

FIG. 1 shows a configuration of a TDM+WDM FBG sensor system according tothe present invention. The ring cavity consists of a gain modulateddevice (GMD) 10 driven by a pulse generator 20 which can switch on theGMD 10 at different repetition rates (frequencies). The output of theGMD 10 is split by a coupler 12 where one of its output acts as theoutput port of the system, while the other output is fed to port 1 of acirculator 14 which directs the signal to an FBG sensor array (G₁₁, . .. , G_(1M), . . . , G_(ij), . . . , G_(N1), . . . , G_(NM)) via port 2,where 1≦i≦N and 1≦j≦M. N is the number of group of FBGs in the array,and each group has M FBGs with different wavelengths. Therefore, thetotal number of FBGs in the array is N×M. Each group of FBGs isseparated by a delay line with length ≧L_(D). The signal reflected bythe FBGs return to the input port of the GMD 10 through port 3 of thecirculator 14, hence forming a ring cavity 16, the circulator alsoensures unidirectional operation of the ring cavity 16.

In this configuration, when the GMD 10 is activated by an electricalpulse from pulse generator 20, a broadband optical pulse is generated atthe output of the GMD 10 and part of the power is sent to the FBG array.Each FBG reflects part of the incident pulse at different time back tothe input of the GMD 10. Those reflected pulses arriving at the GMD 10when it is switched on are amplified while other pulses are absorbed.When the GMD 10 is driven by a periodic pulse train of the pulsegenerator 20 having a period (the time between two pulses) equal to thecavity's round trip time formed by one group of the FBGs (havingdifferent wavelengths), the pulse reflected by this group of FBGs willbe amplified each time it passes through the GMD 10. Hence, by passingthrough the GMD 10 a sufficient number of times, the signal generated bythis group of FBGs at the output port of the coupler 12 can be ofsuitable strength for accurate measurements. Different group of FBGs inthe array can be interrogated separately by changing the pulsefrequency. The driving frequency (f_(i)) and pulse-width (τ) of thesignal to address the i-th group of FBGs in the array are calculatedaccording to equation (1) and (2) shown in FIG. 10, respectively, wherec is the speed of light, n is the refractive index of the fiber,L_(ring) is the fiber length of the ring cavity, L_(i) is the fiberlength from port 2 of the circulator to the middle of a group of FBG andL_(G) is the fiber length that spanned a group of FBGs.

FIG. 2-FIG. 6 represent some examples of the sensor system according tothe present invention. In FIG. 2, a semiconductor amplifier (SOA) 10 ais employed as gain modulated device (GMD) 10. The SOA 10 a is asmall-size semiconductor device that amplifies an input optical signalwhen electric current is applied. The gain region (wavelength) of an SOAis dependent on the material used to fabricate the SOA. As shown in FIG.3, a linear optical amplifier (LOA) 10 b may be employed as the GMD 10for practicing the present invention. The characteristic of an LOA issimilar to that of an SOA except it provides a relative linear amplifierfactor over a wide wavelength region. In FIG. 4, an Erbium doped fiberamplifier (EDFA) 18 coupled with an electro-optical modulator (EOM) 10 care used as the GMD 10. Unlike SOA 10 a/LOA 10 b, the EDFA 18 is anoptical fiber based amplifier which cannot be directly modulated byelectrical pulse, and EOM 10 c is therefore required to close/open thering cavity in this configuration. FIG. 5 illustrates that two SOAs 10 dand 10 e may be employed in the sensor system of the present invention.The two SOAs 10 d and 10 e operating at different wavelength regions canbe cascaded to widen the operation (wavelength) region of the system. Infact, two or more GMDs 10 can be incorporated in this way to enhance theperformance and capability of the system. FIG. 6 shows a differentconfiguration where the coupler 12 is used for both signal output aswell as feedback of the sensors signal to the ring cavity 16. Anadditional isolator 22 is deployed in FIG. 6 in order to ensureunidirectional propagation in the ring cavity 16. The combined functionof the coupler 12 and the isolator 22 in the ring cavity 16 in FIG. 6 issimilar or equivalent to that of the circulator 14 used in FIG. 1.

All the components used in the sensor systems of FIG. 1-FIG. 6 arereadily available commercially, and their functions, capacities,features and other relevant attributes are generally appreciated bypeople with ordinary skill in the art and require no furtherdescriptions herein. General requirements, functional equivalents andsome commercial sources, although generally known to people in thefield, are provided in the following for easy reference.

GMD 10: Any optical device or module that can provide optical gain aswell as shutter functions may be used as GMD 10. Some examples are SOA10 a, LOA 10 b, and EDFA 18 coupled with EOM 10 c, shown in FIGS. 2, 3and 4, respectively. SOA may be obtained from Samsung (www.samsung.com),Qphotonics (www.qphotonics.com/home.php?cat=29), or CIP(www.ciphotonics.com/cip₁₃ products_SOA.htm). LOA may be obtained fromFinisar Corporation (www.finisar.com/optics/LinearOpticalAmplifier.php).EDFA may be obtained from JDSU (www.jdsu.com) or Amonics Ltd.(www.amonics.com/edfa.htm). EOM may be obtained from JDSU(www.jdsu.com).

Pulse Generator 20: any device which generates electrical pulses withadjustable parameters, namely pulse repetition rate, frequency, pulsewidth, rise time, fall time, and the high- and low-voltage levels of thepulses may be used as pulse generation for the purpose of practicingthis invention. A pulse generator may be obtained from Standard ResearchSystem, Model DG535 (www.srsys.com/products/DG535).

Coupler 12: any passive optical devices that connect three or moreports, splitting optical power from one input end to two or moreoutputs, or combining two or more input ends into one output end may beused as coupler for the purpose of practicing this invention. A couplermay be obtained from JDSU.

Isolator 22: any passive optical devices that allow light to propagatethrough it in one direction only may be used as isolator for the purposeof practicing this invention. An Isolator may be obtained from JDSU(http://products.jdsu.com/assets/public/pdf/10139957₁₃005_(—)101405_pfi_ssll_ds_cc_ae.pdf).

Circulator 14: any passive optical devices that connect three or moreports in such a way that allow optical signal propagation from one portto the next sequential port only (that is from port 1 to port 2 but notto other ports, from port 2 to port 3 but not to other ports, and soon). A Circulator may be obtained from JDSU(http://products.jdsu.com/assets/public/pdf/CIR_(—)080805.pdf).

FBG sensor: any passive optical devices that have a periodically alteredrefractive index to reflect certain wavelengths while allowing others topass can be used as FBG sensor for the purpose of practicing thisinvention. FBG sensors can be obtained from QPS Photronics(www.qpscom.com).

Particular Embodiments

(a) Sensing Signal Strength versus FBG Reflectivity

The sensitivity of the proposed ring cavity was evaluated with avariable attenuator followed by a ˜95% reflection FBG to simulate FBGswith different reflectivity, as shown in the inset of FIG. 1. The SOAused in this experiment has a small signal gain of 25 dB and saturatedoutput power of +7 dBm at a driving current of 150 mA. The centerwavelength and optical bandwidth are 1540 nm and 40 nm, respectively.The SOA was biased at 30 mA and modulated with 120 mA by a currentdriver (Analog Devices. AD9662) with pulse rate and pulsewidthcontrolled by the pulse generator (Standard Research Systems. DG535).The coupler fed back 80% of the power to the cavity, although otherpercentages may also provide satisfactory results. About 220 meters ofsingle mode fiber was inserted in the cavity to allow for the 1-MHzmaximum frequency of the pulse generator. The length, of course, shouldbe adjusted according to particular situations. 15-ns pulses (τ) wereused to emulate a fiber delay line of >1.5 meters. The isolator wasemployed to prevent unwanted reflection to the cavity. The output signalis measured by an optical spectrum analyzer (OSA) with a resolution of0.06 nm. FIG. 7 shows the optical power and extinction ratio of theoutput signal as the attenuation is increased. The average output powerand extinction ratio of −17 dBm and 40 dB, respectively were measuredwhen no attenuation was inserted between the circulator and the highreflection FBG. The peak power of the output pulse is thus over +3 dBm(insertion loss of the VOA=0.6 dB and the ON/OFF ratio ˜1/70). Inset ofFIG. 7 shows the output spectra at different attenuation settings. Asthe attenuation increases, the ASE (amplified spontaneous emission)power increases and the average power decreases. With 10 dB attenuation,which corresponds to a FBG with <1% reflection, the noise floor israised by ˜8 dB. However, high quality signal with average power of −24dBm and extinction ratio of 27 dB can still be obtained. These resultsindicate that this configuration is capable of interrogating FBGs withlow reflectivity and hence, a large number of FBGs can be multiplexed ina single strand of fiber.

(b) Single Wavelength TDM System

The system was then tested by connecting an array of nine FBGs havingnearly identical Bragg wavelengths of 1535˜1536 nm and reflectivity of1˜4%, and ˜2 meter long fiber delay lines were used to separate eachFBG. A high reflection (˜90%) FBG with Bragg wavelength of 1540.5 mn wasalso connected at the end of the array to evaluate the crosstalk inducedby the sensing signal when a strong reflection from different wavelengthis introduced. The SOA was driven by the same current condition andpulsewidth as described in the preceding section. The output frequencyof the pulse generator was tuned to address each FBG in the arraysequentially and the output signal measured by the OSA FIG. 8 shows thecaptured spectra of different FBGs where the frequency of the pulsegenerator varied from 896.7 kHz to 804.2 kHz. The spectra produced bythe low reflectivity FBGs show similar output characteristics and nocrosstalk located at the wavelength of the high reflectivity FBG wasobserved. Variations of the output power and extinction ratio of thesespectra are due to different reflectivity of the FBGs. The frequency wasthen fixed at 821.4 kHz that match the repetition rate of the eighth FBGwhile axial strain was applied to tune its wavelength. The inset in FIG.8 shows the change of output spectra of the FBG as a function of appliedstrain, the peak wavelength change exhibits excellent linearity with arelationship of ˜1.1 pm/με up to a level of 4000 με, and no significantchanges in the output power, extinction ratio and spectral shape wereobserved.

(c) Simultaneous Multi-wavelength TDM and WDM System

For a simultaneous TDM and WDM operation, an FBG array comprising fourgroups of three FBGs which have reflectivity of ˜4% and Bragg wavelengthof 1544 nm, 1551 nm, and 1559 nm was constructed. The length of fibersseparating each group and that separating the FBGs within a group were˜6 m and ˜4 m respectively. 40-ns pulses were used to interrogate eachgroup sequentially, FIG. 9 (a)-(d) show the measured spectra when thedriving frequency of the pulse generator were 962.8 kHz, 920.1 kHz,883.6 kHz and 847.4 kHz, respectively. The spectra from each group showsimilar output characteristic. Higher signal power at the second FBG(1551 nm) within each group was observed. This is probably because thepulses reflected from FBGs located near the center of the groupexperience a longer period of amplification each time it passes throughthe SOA. A small spectral peak due to wave-mixing effects in the SOAappeared occasionally. However, its amplitude is very small (<40 dBcompare to the sensor signals) and can be completely eliminated duringthe wavelength measurement process.

The above embodiments of a TDM+WDM FBG sensor system according to thepresent invention, using a pulsed GMD connected in a ring cavityconfiguration, show that the system with low component count and lowloss improves the power and extinction ratio of the sensing signalssignificantly. Satisfactory results were obtained from both TDM andTDM+WDM operations. The system therefore is particularly useful forsensor applications where large numbers of FBG are needed.

While there have been described and pointed out fundamental novelfeatures of the invention as applied to a preferred embodiment thereof,it will be understood that various omissions and substitutions andchanges, in the form and details of the embodiments illustrated, may bemade by those skilled in the art without departing from the spirit ofthe invention. The invention is not limited by the embodiments describedabove which are presented as examples only but can be modified invarious ways within the scope of protection defined by the appendedpatent claims.

1. A sensor system, comprising: (i) a plurality of FBG sensors; (ii) aring cavity comprising a gain modulated device and an optical coupler,said gain modulated device having an input end and an output end; and(iii) a wavelength measuring system.
 2. The sensor system of claim 1,further comprising a pulse generator connected to said gain modulateddevice.
 3. The sensor system of claim 2, further comprising an opticalcirculator with at least three ports connected to said optical coupler,said input end of said gain modulated device and said FBC sensors,respectively.
 4. The sensor system of claim 2, wherein said opticalcoupler has at least four optical leads connected to said input end ofsaid gain modulated device, said output end of said gain modulateddevice, said wavelength measuring system and said FBG sensors,respectively.
 5. The sensor system of claim 3, wherein said opticalcoupler has at least three optical leads connected to said output end ofsaid gain modulated device, said wavelength measuring system and saidFDG sensors, respectively.
 6. The sensor system of claim 5, furthercomprising a first isolator between said optical coupler and saidwavelength measuring system and a second isolator between said opticalcirculator and said input end of said gain modulated device.
 7. Thesensor system of claim 4, further comprising a first isolator betweensaid optical coupler and said wavelength measuring system and a secondisolator between said optical coupler and said input end of said gainmodulated device.
 8. The sensor system of claim 5, wherein said gainmodulated device is a semiconductor amplifier.
 9. The sensor system ofclaim 5, wherein said gain modulated device is a linear opticalamplifier.
 10. The sensor system of claim 5, wherein said gain modulateddevice comprises an Erbium doped fiber amplifier coupled with anelectro-optical modulator.
 11. The sensor system of claim 5, whereinsaid cavity ring further comprises additional one or more gain modulateddevices.
 12. The sensor system of claim 5, wherein said FBG sensors arelocated in a number of sensor groups and sensors within a given grouphave different resonant wavelengths and are configured forwavelength-division-multiplexing based interrogation while sensors amongdifferent groups may have substantially identical resonant wavelengthsand are configured for time-division-multiplexing based interrogation.13. The sensor system of claim 5, wherein each of said FBG sensors hassubstantially identical resonant wavelengths.